A Certain Scientific Truth
by sora-to-umi
Summary: Manami made one last secret promise with Koro-sensei before he dies, that no one else in class 3-E knows about until Itona and the others find out…
1. Chapter 1

Song of the chapter: Orion by Kenshi Yonezu

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"One extra spicy yuzu ramen please!" happily ordered the raven-haired bespectacled girl as soon as they got seated in front of Muramatsu.

"I never took you for a spicy type, guess I was wrong." murmured Itona with the smallest hint of a smirk at the corner of his mouth.

"Nothing beats an extra hot and spicy ramen with extra fishcake and onsen egg!" replied Manami undeterred.

"Like you're the one to talk Itona, it was you who started to take her here in the first place. You couldn't even hide how happy you-" Terasaka was interrupted halfway with a fried gyoza being shoved straight into his mouth by none other than the silver-haired boy. The former started choking and hurriedly grabbed his glass of water to drown down the food into the right pipe. Muramatsu, Yoshida, Takebayashi and Manami were watching their usual antics. Despite being alarmed at first by the way these hot-headed friends communicated with each other, Manami has now gotten so used to it that she didn't beat an eyelash even when furnitures started flying around the little restaurant. She could just roll her eyes at their exclamations, well mostly Terasaka's that true men talked with their fists.

"Here you go Okuda-chan, one extra spicy yuzu ramen right up!" smiled Muramatsu while putting the bowl down in front of the chemist lover.

"Thanks so much Muramatsu-kun! Your ramen is always the best!" cooed Manami while proceeding to take her chopsticks to slurp down and the delicious noodles.

After their fateful third year of middle high school, the students of class 3-E parted ways with their entrance into different high schools. However, some groups of students ended up going to the same high school because of their similar interests in study fields. As Itona, Manami and Takebayashi wanted to go into a sciences-specialized school, they all chose the most famous sciences- oriented school in Tokyo, Oku Tokyo Kagaku Gijutsu. Yoshida also enrolled in the same school because of his interest in mechanics and vehicle. Terasaka decided to enter a school near there, where he can focus on social studies to become a politician. Muramatsu wanted to inherit his dad's ramen shop, so he chose a 2-years culinary course in a specialized school in the same prefecture.

Their chosen schools were so close that Manami found herself getting even closer to the whole Terasaka-gang and Takebayashi. She still saw Karma, Kayano and Kanzaki from time to time, even though they lived and studied in other prefectures. Throughout her first months of high school, Manami got more comfortable to Itona, Takebayashi and the others, even more than when they were in junior high to the point that she didn't even stutter around them anymore. She was dubbed 'the glasses witch' by Terasaka, Muramatsu and Yoshida, since Hazama was unofficially considered 'the gloomy witch' of their group back in junior high school. Even if Hazama went to a literature-oriented university in another town, she still showed up at Muramatsu Ramen Restaurant from time to time. She got along pretty well with Manami, despite the latter's being intimidated by Hazama's spooky attitude at first.

Manami, for her part, was really glad to see some familiar faces in high school. When she first started at Oku Tokyo, she was so afraid to be all alone again, with Karma, Kayano and Kanzaki all went to different schools respectively.

Unknown to the bespectacled girl, a pair of golden eyes is intently observing her discreetly. Even though he hasn't really talked much to Manami during their third year of junior high, Itona did take more notice of her than the other girls in class 3-E, mainly because the proximity of their seats and her passion in sciences. Also, in their last fight against the government and Yanagisawa while the latter was trying to trap Koro-sensei in the mountains to kill him, Manami did stay with Itona and the rest of Terasaka's group to get rid of the government's officers who were preventing them to come to their mountain campus to try to rescue their sensei. He remembered using her efficient and easy-to-use smoke bombs and chloroform bullets.

When they first met at Tokyo Oku as fellow classmates for physics and chemistry, it was a bit awkward at first to start the conversation even though they knew each other, mainly because of Manami's shyness and also because Itona was trying to filter what he said since he knew that he could be pretty blunt sometimes and didn't want to scare the girl off. However, in the later periods as the first trimester progressed, Manami got more comfortable around Itona and started to animatedly discuss with him about her favorite subject. He also discovered that the chemistry-lover was also interested about other branches of sciences, such as physics and math. She did admit that she wasn't as good with the latter as she was with chemistry. That, however, didn't stop her competitiveness from trying to get the best marks that she can in them, and not just in chemistry.

Before they knew it, they became project partners in all of their shared classes, helping each other mutually with their own forte while comparing their grades with each other to push themselves further. Itona also invited her to hang out with him and the rest of Terasaka's group, along with Takebayashi who also got close with them since junior high after he introduced Terasaka to the greatness of maid cafes. They usually met up at Muramatsu's Ramen Shop at least three times a week, if not every day after their classes. To their pleasant surprise, they discovered that Manami liked ramen almost as much as she liked chemistry. It was her who gave Muramatsu more helpful tips about how to enrich the ramen's broth, as she claimed that cooking was really similar to chemistry. "It's all about following a recipe and get it right!" she said.

As he kept on discovering more and more interesting facts about Manami, Itona was starting to see why Karma always made a point to keep Manami as his assassination partner during their third year in junior high.


	2. Chapter 2

Sorry for any grammar mistakes and OOC!  
AssClass is not mine unfortunately, if it was, Manami would have clearly ended up with either Itona or Karma at the end.  
Song of the chapter: Amadeus by Itou Kanako (Steins Gate O game opening)

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"Where are you going, Itona? You're not going to Muramatsu's place?" – asked Terasaka, who was waiting for the silver-haired boy at in front of Tokyo Oku after a usual day of school. They were already entering their third week as high school freshmen. Okuda, Takebayashi and Yoshida already went ahead to the ramen restaurant.

"Sorry, I'll pass for today. I just remembered that I have to retrieve the camera drones that I installed inside every room at the 3-E campus. It's been a long time that I forgot to get them back, since when Koro-sen…"

Itona cut himself, a slightly pained expression on his face. He didn't need to finish that sentence for Terasaka to understand what he was talking about. The latter seemed to have the same expression, everyone in 3-E did honestly when they slipped up and refer to their former teacher.

"Okay, I'll tell the others that you have something else planned today. Lend me one of those drones when you get them back will ya? It's been a while since I played with them." Concluded Terasaka.

Itona agreed and the two friends parted way. He quickly retrieved the drones in the hidden place inside their mountain campus. He tried not to stay too long to prevent all the nostalgia and sad feelings swelling up too much in him.

When he got back home that night, the silvered-haired boy browsed through all the videos recorded by his drones' camera before deleting them. However, he paused when a certain raven-haired bespectacled girl appeared in one of the videos.

"Is that… Okuda?" – wondered the mechanics expert. He was almost certain that it was her, even though the video's quality wasn't the best, there was no mistaking those twin braids.

The date was recent, barely two weeks ago. That meant she came back to the abandoned campus not long after the high school opening ceremony. But why? She didn't mention anything about it when we saw each other at school.

In the video, the petite girl seemed to be searching for something inside the sciences lab's drawers. There wasn't enough light in the room so it was hard to see what she was searching for. She finally seemed to find it, whatever it was, and quickly put it in her schoolbag, then exited the building.

It was true that she didn't have to tell him every single thing that was going on in her life, but Itona couldn't help to feel that something was off. From what he saw of Manami until now, she was a sincere and straight-forward girl. She must have had a good reason to come to their old campus like that. While Itona would have let it slide and not bother himself more with it if the person didn't directly ask for his help, he couldn't help but want to do otherwise with the chemistry genius. For some reasons that he couldn't yet explain clearly, he wanted to get more involved in matters concerning her and to lend her a helping hand if it was needed.

Putting the drones away, he decided that he will confront her about it later at school.

Two days passed since his discovery, and it was already Thursday. They were hanging out at Muramatsu's ramen place as usual.

"What were you doing two weeks ago at the mountain campus, Okuda?" – asked Itona wthout beating around the bushes, as he was chowing down his ramen.

It was an unspoken agreement between all ex 3-E students that "mountain campus" means the one that they spent their treasured year with Koro-sensei.

Manami visibly froze, albeit for only mere seconds. Since the start of high school, the chemistry lover has gotten much better at concealing her emotions and thoughts. Instead of wearing them on her sleeves like during their middle high school year, she seemed to take Koro-sensei's advice to heart by trying to improve her talking skills and reactions, and also to read others better. She turned towards Itona and, with the calmest voice that she could muster without stuttering, replied back to him:

"I don't know what you're talking about, Itona-kun."

"Don't play dumb with me. I saw you with my camera drones. You came there two days after the opening ceremony of our school."

"I-I just forgot something at the old campus, so I went there to retrieve it." Flinched Manami in her head as she was stuttering again.

She also got better at lying too, but I can still read her.

He decided to not push it further even though he didn't buy what she said. Knowing her, Manami could be very stubborn at times, and this was one of those times. Instead, he concealed his presence and followed Manami after their classes. He managed to put a chip on her schoolbag without her noticing, which would show her location on his phone's map.

Three days after his confrontation with the raven-haired girl about her being at the mountain campus, the girl finally made a move. He realized that day from the map that she wasn't taking the usual route that she always took to go home. Instead, she was heading towards a more industrial section of the city, near Aoto station. Wasting no time, Itona quickly got home to change out of his school uniform, since he didn't want to draw out more attention while tailing his classmate and wearing a uniform from a famous school.

As soon as he caught up with her signal after finishing changing, Itona started to speed up and was jumping from one building to another to catch her, making use of all his training both while under Yanagisawa and during his time in class 3-E. His reputation as the swiftest student of their class wasn't just for show as he caught up with his target in no time despite Manami having a head start. Itona made sure to conceal his presence well while observing his classmate who was standing in front of him on the same building.

The silver-haired lad's eyes widened a bit as his eyes took in her appearance. Instead of her usual twin braids, Manami has tied her hair up in a high pony tail, hidden under a purple cap. Her usual glasses were also replaced by contact lenses. She was wearing a purple short-sleeved T-shirt, with black leggings reaching her knees topped by a blue jean skirt. If not for his phone's map indicating that Manami was right there, Itona wouldn't have thought that the girl in front of him was his shy partner.

Unaware that her classmate was just barely 20 m behind her, Manami gathered her thoughts together and went through the plan again in her head. She would need to jump down the building where she was currently on top of using a rope in order to land at the window some seven floors below and entered the research facility that she needed to infiltrate. While she wasn't anywhere as good as Itona-kun or Okano-san in terms of agility, Manami's training in class 3-E didn't go to waste and she was able to jump and manoeuvre herself well above a normal person's standard.

The petite girl tried to push back the nervousness that Itona has discovered her coming to the campus the other day. Manami convinced herself that he didn't have any other information and she just had to be more careful to not let anything else slip out from that point.

Manami closed her eyes, feeling the strong evening wind on her skin, and thought about her last promise to Koro-sensei three days before he was confined in the mountain campus by the government:

 _"Okuda-san, after I die, can you help stopping the ongoing research on antimatter by producing the neutralizing chemical that you made for me before?" calmly asked the octopus teacher, though with a small hint of hesitancy, as he was helping his bespectacled student cleaning up the instruments in the sciences lab, after another day of chemistry experiments._

 _"P-Please don't say that sensei. There's less than 1% that you will explode and blow the earth. W-we will try our best to save you. And..T-that research is still going on despite what happened with you sensei?" stuttered Manami with widened eyes._

 _The octopus teacher smiled, with his whole face tinted with a red color with a red circle on top, showing how he was pleased and touched by Manami's concern for him._

 _"Thank you Okuda-san. I'm grateful to be the teacher of all of you." Started Koro-sensei_

 _"And as for your other question, unfortunately yes, that horrible research is still on going. I would prefer not to have one of my students getting involved if it is possible, but the government was already informed that you were able to create that chemical, and the only one able to as that. So they want to get you to do it. I already asked Karasuma-sensei, he would shield and help you as much as he could, you just had to create the chemical and the rest would be taken care of by him." Explained the octopus teacher, who had made Karasuma swore that he won't let the government use his precious student to advance that abdominable research and to not let her get harmed in anyway._

 _"I-I would be honored to help you sensei. Sciences are supposed to help people, and not destroy their lives! I will do my best to stop that research!" declared Manami in a clear voice_

 _"This can get really dangerous. You must promise me, Okuda-san, to not act reckless at any time. The only thing that you need to do is recreate the chemical in a supervised research lab, do not get involved in anything else, okay? I'm sorry to impose such an unreasonable request on you, Okuda-san." The "I don't want to let anyone else's life be destroyed by that experiment just like mine and Aguri's" part was left unsaid, but the bespectacled girl understood._

Finally opening her eyes and positioning herself while holding the rope, Manami repeated in her mind the same thing that she said to her beloved teacher that day, while jumping down from the building at a high speed:

"I won't let sciences destroy people's lives anymore, Koro-sensei!"

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